This invention relates generally to micro-electromechanical systems (MEMS) devices and processes, and more specifically to processes for providing MEMS devices which incorporate top caps or covers with upper sense plates.
One method of producing micro-electromechanical structures (MEMS) is by bonding a patterned silicon wafer to a glass (usually pyrex) substrate. Portions of the silicon wafer are etched away, leaving a mechanical silicon structure anchored to the glass substrate. The process is initiated with a glass wafer. A cavity is formed in the wafer using a wet or dry etching process. A depth of the etch determines a separation between the structure's capacitive elements. Metal layers are deposited and patterned on the glass, forming conductive electrodes and interconnects. A heavily boron doped (p++) epitaxial layer is grown on a separate, lightly doped silicon substrate. A pattern is etched into the silicon wafer to a depth greater than the thickness of the epitaxial layer. The glass and silicon wafers are bonded together using anodic bonding. Using an etchant that etches lightly doped silicon but not p++ silicon, the undoped portion of the silicon substrate is etched away, leaving the freestanding microstructures. Such a process is generally referred to herein as a silicon-glass MEMS process.
In most known MEMS devices, the microstructure is exposed to the ambient environment during operation. At least some of the known problems associated with exposed microstructures are described below. Furthermore, if it is desired to provide a special operating environment, or simply protect the microstructure from the ambient environment, the protection must be done at a packaging step.
One known packaging step is to bond a glass wafer containing recesses, sometimes referred to as an upper wafer or top cap, on top of the previously fabricated glass/silicon wafer, which is then sometimes referred to as a device wafer. At least part of the silicon structure on the device is a continuous support ring that completely surrounds, but may or may not be connected to, the microstructure. The second glass wafer is bonded to the seal ring, forming the cavity. In addition, it is desirable to have electrical leads extending from outside the cavities into the cavities. While such methods and devices have been contemplated, they have not yet been successfully integrated into a manufacturing environment.